The active area of the pad is approximately 2′ x 2′. I think the foam is polyurethane open-cell foam but I’m not sure. It’s the stuff you use to pack shipping crates. The load cells are made by sandwiching the foam in between 1″ circle cutouts of copper-clad FR4 PC board. I used single sided board and a hole saw with the pilot bit removed (use a drill press and a clamp). The capacitance varies as the foam compresses, and the amount of capacitance is directly related to the thickness and density of the foam, as well as the area of the copper conductors (plates). So you can create any size or thickness load cell you want really. A bigger plate results in more capacitance, as does placing the plates closer together. I estimated the capacitance I would have in my application with this calculator I found at Daycounter engineering services.

I created an array of 64 cells by making 8 rows and 8 columns, each with 8 copper-clad discs. Wherever the row/column discs align a load cell is created. An AVR typically has eight available ADC inputs along with another eight control outputs, so this way you can scan down through the rows and columns to measure each cell. A square wave is sequentially output on the columns, and after some analog proccessing the AVR’s ADC scans each row. The analog voltage present represents the amount of pressure (capacitance) at each site.

The square wave generator is the old two op-amp ramp generator/comparator circuit. This is fed to one side of the cell. The output from the other side is coupled through whatever capacitance is available at the cell, and results in a triangle wave with amplitude proportional to the capacitance thanks to a transimpedance amplifier. Then it is fed to a peak detector and amplified. The signal is then sent to the ADC. There are more than a few examples out on the web for capacitance sensing, here is a good one.

The AVR outputs the 64 values on a serial port. I used excel’s surface chart to represent the data. The chart posted here is a map of an english saddle.

It works fairly well, but my analog section needs some improvement. Sensitivity is sufficient for this test, but it’s relatively poor. Response time also suffers because of noise; my prototype could definitely use improvement in that area and I had to decrease the response time of the amplifier to get consistent output. This is unfortunate because I had planned on trying to make a movie by recording real-time data while riding a horse. I even designed it to run on a 9V battery for this purpose. Maybe if I have enough free time next summer (unlikely) I’ll try and tune it up a bit.

It’s a regular halter, with a hard poly type rope integrated into the nose piece and behind the ears. You can connect the lead rope to the regular halter for comfort, but the lead rope can be easily switched to the second ring and the poly rope will bite in all the right places to get the horse’s attention.  It leverages pressure points on both sides of the face and over the poll.  This lends it to stopping horses that want to either pull back or pull away.  After it was completed we placed it on the young gelding finalizing the fit for pressure points.  It worked very well both when he was tied up to stop him from pulling back and also while he was being lunged.  If you are interested in purchasing one, send me an email.

My demo horse Shady